1. Technical Field
The invention concerns a magnetic field sensor with a Hall-element and with a rectangular change-over switch, which switches between connections for the supply current for the Hall-element and connections for the Hall-voltage, dependent on a timing signal, and with a filter circuit, which filters the signal delivered by the rectangular change-over switch.
2. Discussion
A magnetic field sensor of this kind is known from DE-A44 31 703 by the same applicant. Because of the rectangular change-over switch the offset voltage occurring at the Hall-element is largely eliminated in the temporal average, whereby the measurement accuracy is increased overall. One disadvantage with the known magnetic field sensor is that because of the rectangular change-over switch the output signal is not exactly constant over time, but rather that there arises at switchover points in time a discontinuity of the course, because the offset voltage overlays the Hall-voltage in a rectangular shape and at switchover points the operational status is not defined for brief periods because of ever present switching differences. The output signal is thus faulty at least at these switchover points in time and cannot be evaluated. There are at present numerous cases of applications wherein the output signal of the magnetic field sensor is to be evaluated time-continuously, for example the tooth-flank detection of crankshafts or camshafts in the automobile. With such applications, the output signal of the magnetic field sensor is to be processed with a high time resolution.
It is the task of the invention to offer a magnetic field sensor whose output signal is continuous over time and illustrates the magnetic field apprehended by the Hall-element with great precision.
The invention uses as a filter circuit a ladder-type filter, where the capacitor contained at each stage is charged in the first timing phase with the difference signal amplified if need be, and in the second timing phase with the inverted difference signal. As a result of the rectangular switchover, the useful signal contained in the difference signal, which corresponds to the magnetic field apprehended, is also mutually inverted in both timing phases. Accordingly, the common mode signals in the difference signal which overlay at each filter stage because of the electronic components, e.g. offset voltages, are equalized in the temporal average. Such common mode signals which occur are thus altogether eliminated, whereas the useful signal delivered by the rectangular change-over switch is, if need be in an amplified form, reproduced with great precision. By means of the ladder-type filter low pass filters of low cut-off frequency can be achieved. Consequently, the signal produced by the magnetic field sensor and full of discontinuities resulting from the chopper operation can be given off continuously over time and be further processed.
An embodiment of the invention uses an active ladder-type filter as a filter circuit. A ladder-type filter of this kind is described for example in IEEE J. Solid-State Circuits, vol. SC-26, no. 12, pp.1988-1997, December 1991. A ladder-type filter of this kind has the advantage that it has exceptional filtering characteristics, and can be produced relatively simply in CMOS technology. As amplifying elements preferably OTA (operational transconductance amplifier) modules can be used, which work in gyrator switching. These OTA modules are also designated as transconductance amplifiers. The gyrator switching makes possible the transformation of an impedance into the dual impedance thereof. By means of the gyrator switching inductive components of a filter with use of capacitors can be achieved. In this way, the ladder-type filter can be made up in small construction size as monolithically integratable circuit, although to a considerable extent it contains inductive aspects, which as a passive LC ladder-type filter would require a considerable construction volume in order to create the desired filtering performance, but could certainly not be represented monolithically.
It is well known that OTA modules have a considerable offset component, whereby the output signal of the active ladder-type filter contains overall a considerable offset voltage. An offset voltage of this kind would destroy again the measurement precision of the apprehension of the magnetic field achievable by the rectangular switchover. According to the invention, OTA modules connected in series in the ladder-type filter are connected through their inlets and outlets in a chopper operation in such a way that the offset voltages occurring, which show themselves as voltages at the capacitors, compensate in successive timing phases. For this, the outlet voltages of the OTA modules are fed to the capacitors alternately, dependent on the timing signal, i.e. in the first timing phase via the inverting outlet to the first capacitor and in a second timing phase via the same inverting outlet to the second capacitor. The non-inverting outlet is connected correspondingly. In this way, a chopper operation is achieved, wherein the offset voltages of the OTA modules are compensated and no longer occur in the output signal of the ladder-type filter. This ladder-type filter creates a time-continuous output signal, the frequency of the timing signal being suppressed by the low pass performance of the ladder-type filter. Accordingly, a voltage exists at the outlet of the ladder-type filter which has no discontinuities even at the points in time of the switchover of the rectangular change-over switch, and can be processed further with high time resolution. The great precision of the illustration of the magnetic field size apprehended by the Hall-element achieved by the rectangular change-over switch is retained.